Printed circuit boards (PCBs) have become increasingly complex in the last few years. Physical board dimensions and higher interconnect densities have required rigorous manufacturing and process controls on PCBs. Plated through holes (PTHs) have been an area of particular concern in the manufacture and reliability of PCBs.
Furthermore, a variety of new manufacturing processes such as surface mount technology, and wave soldering subject PCBs to increased thermal stresses which may have an adverse impact on plated through hole reliability.
In general, it has been found that the main cause of a PTH failure may be traced to significant differences in thermal expansion between the copper barrel lining the hole and the surrounding substrate. When the differential expansion causes strain that exceeds the strength of the copper barrel, a rupture intermediate its ends may develop due to thermal stresses. This usually results in open circuits or intermittent contacts which may lead to system failure.
In general, thermal shock or slow thermal cycling tests are employed to determine the capability of the PTHs to withstand cyclic temperature variations encountered in actual field life. Some of these are international standard tests as for example, IEC oil shock test, fluidized sand bath test, MIL-P55110 thermal shock test, MIL-STD-202 temperature cycling test, and APD-oil shock test. In these tests, the PCB specimens are subjected to temperature excursions from -65.degree. C. to +125.degree. C. or 25.degree. C. to 260.degree. C. at different rates of temperature changes. Changes in resistance of a series of interconnected plated holes beyond some arbitrary fixed value after a test is generally considered a failure.
In a study (IPC Round Robin) published September 1988 on reliability of small diameter holes, it was established that 400 cycles of MIL-T-thermal cycling, between -65.degree. C. to +125.degree. C., with 30 minutes of dwell time at each temperature extreme is the most discriminating test to distinguish between holes of good and poor reliability showing up both early life and wear out failures. Temperature cycling simulates the diurnal temperature variations experienced by the end product during a its field life. Failures occur when the fatigue life of the plated conducting barrel of a plated-through hole or that of a foil pad is exceeded.
Thermal cycling tests are time consuming, need elaborate chambers, and are often expensive to perform. In MIL-STD testing, the whole PCB specimen is uniformly heated to the desired temperature of 125.degree. C. in a thermal chamber. Between 15-30 minutes are allowed for soaking and stabilization. In order to perform an adequate number of cycles this testing requires weeks to perform.
It is therefore an object of the invention to provide a method for testing the reliability of plated-through holes on a printed circuit board which is less time consuming and less expensive than the known methods of temperature cycling mentioned above.